1. Field Of The Invention
The present invention relates to a composite overhead cable structure for electric and optical transmission, containing a welded metallic pipe and an optical fiber cable disposed therein, and a process for producing the same. More particularly, the present invention is concerned with a composite overhead cable structure for electric and optical transmission, comprising a metallic pipe, at least one layer of stranded metallic elongated conductor surrounding the outer periphery of said pipe in close contact with said outer periphery, and an optical fiber cable disposed within and extending throughout the length of said pipe, wherein said metallic pipe comprises at least two metallic pipe sections arranged in alignment and connected at their respective confronting terminals having contours complementary to each other, and has a unique structure of weld connection between the mutually adjacent metallic pipe sections, so that high seal properties are imparted to the composite overhead cable structure. The above metallic pipe has no weld defects in the longitudinal weld thereof, thereby enabling the metallic pipe to be free from the danger that rain water or the like enters the pipe and adversely affects the optical fiber cable disposed therein. The present invention is also concerned with a process for producing the above-mentioned composite overhead cable structure for electric and optical transmission.
2. Discussion Of Related Art
Heretofore, there have been known composite overhead cable structures for electric and optical transmission in which an optical fiber cable and an overhead cable are combined with each other. FIG. 1 shows the cross-section of one example of such composite overhead cable structures for electric and optical transmission. In the composite overhead cable structure of FIG. 1, metallic pipe 1 generally having a wall thickness of 1 to 2.5 mm and an outer diameter of 5 to 12 mm and made of aluminum or the like is disposed within stranded conductor 2 which functions as a ground wire. An optical fiber cable 3 is disposed inside metallic pipe 1. The welded metallic pipe employed in the conventional overhead cable structure is shown in FIG. 2. Metallic pipe 1 of FIG. 2 contained in the conventional composite overhead cable structure is prepared from metallic tape 1' of FIG. 3.
A composite overhead cable structure having the structure as shown in FIG. 1 is generally produced by the steps of (1) bending metallic tape 1' of FIG. 3 having a thickness of 0.8 to 2 mm into a substantially cylindrical shape successively from one terminal of the metallic tape to the other terminal so that opposite longitudinal side edges of the metallic tape are brought into abutment against each other to form a substantially cylindrical preliminary pipe, while inserting an optical fiber cable into the substantially cylindrical preliminary pipe being formed from metallic tape 1', (2) welding together the longitudinal side edges, which have been brought into abutment, of the preliminary pipe, (3) squeezing the resultant metallic pipe through a die or the like to extend the pipe while reducing the diameter of the pipe, thereby obtaining a metallic pipe having the optical fiber cable disposed therein, and (4) stranding a metallic elongated conductor around the outer periphery of the metallic pipe.
As is understood from the production process described above, the length of a composite overhead cable structure depends on the lengths of the materials, namely, the optical fiber cable, metallic elongated conductor and metallic tape (to be used for forming the metallic pipe). At present, the available lengths of a metallic conductor and an optical fiber cable are at most 50 km and 7 to 8 km, respectively. With respect to a metallic tape, the available length is at most 4 km. The unit length of a composite overhead cable structure is determined depending on the length of the material having a length which is shortest among all the materials, namely, the length of the metallic tape (at most 4 km). In recent years, however, according to the progress of networking using optical transmission, the demand for composite overhead cable structures having increased lengths has been increased. For example, in the United States, a length as large as 6 km or more has been desired for composite overhead cable structures.
For providing composite overhead cable structures having increased lengths, it is necessary that a plurality of metallic tapes be welded together at their terminals to form an extended tape having a desired length.
With respect to the conventional method for welding the terminals of metallic tapes for obtaining an extended metallic tape having a length as large as 6 km or more, an explanation is made below, referring to FIGS. 2 and 3. Two metallic: tapes 1A' and 1B' are welded at their terminals each having a contour of a straight line vertically extending between both edges of the tape. The formed weld is designated by W.sub.1 in FIG. 3. However, with respect to the thus welded metallic tape 1', there is a problem such that when the welded metallic tape 1' consisting of metallic tapes 1A, and 1B, is bent into a substantially cylindrical shape so that opposite longitudinal side edges of the bent tape abut each other and welded at the longitudinal side edges to obtain a metallic pipe, defective welding is likely to occur between the longitudinal side edges at a point where longitudinal weld W.sub.2 crosses circumferential weld W.sub.1, thereby forming gap G, as shown in FIG. 2.
The reason for the occurrence of defective welding at the point where longitudinal weld W.sub.2 crosses circumferential weld W.sub.1 is as follows. When metallic tape 1' is bent into a substantially cylindrical shape, both ends of weld W.sub.1 of extended tape 1' are caused to meet each other. However, since the stiffness of weld W.sub.1 is greater than the stiffness of other portions of metallic tape 1', the bending of metallic tape 1' into a substantially cylindrical shape to form a pipe cannot be completely performed at weld W.sub.1, causing the abutment of both the ends of weld W.sub.1 to be poor. Consequently, even when it is tried to effect welding along the longitudinal side edges of the bent tape so as to form weld W.sub.2, both the ends of weld W.sub.1 which are in poor contact with each other cannot be completely welded together, leaving gap G, as shown in FIG. 2. That is, the so-called cross-welding as shown in FIG. 2 cannot provide complete welding at the cross-point.
In the conventional composite overhead cable structure using a metallic pipe having the abovementioned weld defects, i.e., gap G, it is likely that rain water or the like disadvantageously enters the metallic pipe through the gap and adversely affects the optical fiber cable housed therein, leading to a lowering of transmission properties of the optical fibers in the optical fiber cable.